ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-13-129-2013Transport analysis and source attribution of seasonal and interannual variability of CO in the tropical upper troposphere and lower stratosphereLiuJ.1LoganJ. A.1MurrayL. T.1PumphreyH. C.2SchwartzM. J.3MegretskaiaI. A.11School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA2School of Geosciences, The University of Edinburgh, Edinburgh, UK3NASA Jet Propulsion Laboratory, Pasadena, CA, USA08012013131129146This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/129/2013/acp-13-129-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/129/2013/acp-13-129-2013.pdf

We used the GEOS-Chem chemistry-transport model to investigate impacts of
surface emissions and dynamical processes on the spatial and temporal
patterns of CO observed by the Microwave Limb Sounder (MLS) in the upper
troposphere (UT) and lower stratosphere (LS). Model simulations driven by
GEOS-4 and GEOS-5 assimilated fields present many features of the seasonal
and inter-annual variation of CO in the upper troposphere and lower
stratosphere. Both model simulations and the MLS data show a transition from
semi-annual variations in the UT to annual variations in the LS. Tagged CO
simulations indicate that the semi-annual variation of CO in the UT is
determined mainly by the temporal overlapping of surface biomass burning
from different continents as well as the north-south shifts of deep
convection. Both GEOS-4 and GEOS-5 have maximum upward transport in April
and May with a minimum in July to September. The CO peaks from the Northern Hemisphere (NH) fires
propagate faster to the LS than do those from the Southern Hemisphere (SH) fires. Thus the transition
from a semi-annual to an annual cycle around 80 hPa is induced by a
combination of the CO signal at the tropopause and the annual cycle of the
Brewer-Dobson circulation. In GEOS-5, the shift to an annual cycle occurs at
a lower altitude than in MLS CO, a result of inadequate upward transport. We
deduce vertical velocities from MLS CO, and use them to evaluate the
velocities derived from the archived GEOS meteorological fields. We find
that GEOS-4 velocities are similar to those from MLS CO between 215 hPa and
125 hPa, while the velocities in GEOS-5 are too low in spring and summer.
The mean tropical vertical velocities from both models are lower than those
inferred from MLS CO above 100 hPa, particularly in GEOS-5, with mean
downward, rather than upward motion in boreal summer. Thus the models' CO
maxima from SH burning are transported less effectively than those in MLS CO
above 147 hPa and almost disappear by 100 hPa. The strongest peaks in the CO
tape-recorder are in late 2004, 2006, and 2010, with the first two resulting
from major fires in Indonesia and the last from severe burning in South
America, all associated with intense droughts.